The automobile manufacturing industry is constantly faced with new challenges in a wide array of areas including vehicle safety, reliability, durability, and cost. Perhaps the greatest challenge faced by the automobile industry today is the need to improve fuel mileage to both decrease carbon emissions and increase fuel economy for both environmental and cost reasons, all without compromising safety, power, or durability. In 2011, new fuel economy requirements were imposed that establish a US vehicle fleet average of 54.5 miles per gallon by 2025. As the industry moves to that target year, fuel annual economy requirements will be ramped up for different-sized vehicles.
Efforts have been made to increase fuel economy for vehicles. These efforts can be divided into two approaches: the “supply” side and the “demand” side.
On the supply side, attention is drawn to improving energy conversion efficiency through use of, for example, electric or hybrid-electric drive trains. In addition, new vehicle drive trains, including smaller engines and more efficient transmissions having multiple gears and transfer cases, are being developed and employed. Other technologies, including start-stop and engine cylinder deactivation strategies, are also proving effective at decreasing fuel consumption. Improved transmissions with multiple gears are also important elements to increased fuel consumption efficiencies.
On the demand side, weight reduction is key, though other aspects, such as improved aerodynamics and drag reduction, are also important. Conventional vehicles, particularly trucks, rely on steel components. For over 100 years the material of choice for most vehicles is steel. Today steel makes up about 60% of the average car by weight.
Despite the improvement in steel composition the weight of steel regardless of type remains significant. It is also possible to reduce vehicle weight when steel is used by reducing component thickness. However, at a certain point it is no longer practical to reduce steel thickness regardless of the steel grade used. The use of high strength steel or advanced, high strength steel does not improve the realization that there are limits to how much vehicle weight can be reduced by steel thickness reduction without compromising vehicle performance.
Thus as the automotive industry continues to focus on light weighting vehicles to meet customer expectations on fuel economy and CAFE requirements, interest in alternative materials including aluminum intensive vehicle applications has increased. This is because vehicle weight reduction is most directly accomplished through substituting lighter materials for currently used steel parts. However, a limited variety of materials are available as a substitute for automotive steel. One such material is carbon fiber which is both lightweight and strong.
While carbon fiber offers certain performance advantages, replacement of the steel body-in-white with carbon fiber is expensive and brings with it a relatively slow production process.
Accordingly, much attention is drawn to the use of aluminum which is about ⅓ the weight of steel. Aluminum is not a new material for automotive use and has been used as a material for castings for over 100 years. The use of aluminum not only provides weight reduction but also results in good crash performance. Research has shown that in collisions aluminum can perform as well as conventional steel and demonstrates the ability to absorb twice the crash energy per pound of mild steel, having good buckling and energy absorption characteristics.
In body-in-white structures, joining methods have traditionally relied on resistance-spot welding (e.g., in steel structures). In the case of aluminum intensive vehicles and other mixed metal joining applications, self-piercing rivet (SPR) technology prevails. One advantage of SPR technology is that it is a high production volume assembly process. Further, it is compatible with an adhesive, where both methods can be used in conjunction.
The challenge often faced with SPR, however, is that the substrate material may be difficult to pierce. This can result in rivet fracture or buckling, thereby compromising joint integrity. Moreover, upon riveting the substrate material may accumulate damage which is undesirable for durability resistance. One example of this is fiber delamination in composite materials. Lastly, corrosion concerns can be introduced when a large galvanic potential exists between the rivet material versus the substrate material. This can degrade the joint integrity with time and exposure to ambient environmental conditions.
As in so many areas of vehicle technology there is always room for improvement related to the mechanical fastening of the materials through self-pierce riveting.